In most drilling operations, cement contamination occurs one or more times when casing strings are cemented and the plugs are drilled out. The extent of contamination and its effect on mud properties depends on several factors. These include: solids content, type and concentration of deflocculants, and the quantity of cement incorporated. One 94-lb sack of cement can yield 74 lb of lime. When cement is completely cured only about 10% is available; whereas, when it is soft (green) as much as 50% of the lime may be available to react. It is the calcium hydroxide (lime) in cement, reacting with solids, that causes most of the difficulty associated with cement contamination.
Freshwater bentonite systems are flocculated by cement, resulting in increased rheology and fluid loss. The severity of flocculation depends upon the quantity and quality of solids present and the solubility of the Ca++ ion. Therefore, when cement contamination reaches a level where it is no longer practical to treat out, it may be desirable to convert the system to a calcium-based mud. It may be possible to isolate the contaminated fluid and dump it.
Treatment - To maintain a low calcium drilling fluid, chemical treatment must be used to remove cement contamination. The aim of treatment is to control pH while removing calcium and excess lime from the system as an inert, insoluble calcium precipitate. The treatment of choice for cement or lime contamination is sodium bicarbonate (NaHCO3). In its reaction with lime, a hydrogen ion is liberated which reacts with a free hydroxyl ion to form a water molecule. This reaction serves to further reduce the pH and allow more lime to go into solution. The lime and sodium bicarbonate reaction is: Ca++ + 20H- + Na+ + H+ + CO3 = ^ CaCO3 + Na+ + OH- + H2O.
To chemically remove 100mg/L of calcium originating from lime would require approximately 0.074 lb/bbl of bicarbonate of soda. If lignite is present in the system, it will also react with lime to form a calcium salt of humic acid. One lb/bbl of lime will react with 7-8 lb/bbl of lignite. Calcium salts of humic acids will create viscosity problems at higher concentrations.
^^ Note: Lignite is not an acceptable treatment for cement contamination
Chemical removal of calcium ions with bicarbonate ions does not normally correct the damage done to a dispersed system. It is usually necessary to treat with deflocculants to obtain desired rheological properties. Materials, such as prehydrated bentonite, CMC or PAC are used to restore filtration and filter cake characteristics.
Pretreating presents a problem, since it is difficult to predict the extent of contamination prior to drilling the cement. Overtreating with bicarbonate of soda could be as detrimental to drilling fluid properties as the cement contamination. Therefore, it is not advisable to pretreat with more than 0.5 lb/bbl of bicarbonate of soda. One approach to avoid overtreatment is to treat only soluble calcium and wait to treat lime when it goes into solution on subsequent circulations. Treatments should be discontinued when excess lime approaches 0.3 to 0.5 lb/bbl. Materials such as lignosulfonate and lignite are good supplementary pretreating agents because they buffer the pH and aid in deflocculating the system.
Note: SAPP (sodium acid pyrophosphate) is sometimes used to treat out cement; however, it will simultaneously thin the system. However, above about 180°F it can thermally degrade to orthophosphate which is a flocculant.
Consideration should also be given to low-gravity solids content prior to drilling cement, because high clay solids concentration is a primary cause of flocculation when cement contamination occurs. Reduction of solids concentration, if too high, is recommended as a defense against severe floccula-tion.
Because pH values are high when drilling cement, the quantity of calcium ion in solution rarely exceeds 300 to 400 mg/L. For this reason, much of the cement drilled remains as discrete particles and is available to dissolve and replace the calcium ion that has been treated out of solution. Unless cement particles (excess lime) are mechanically removed from the system by efficient solids removal equipment, it will take several days for the excess lime to solubilize, react and be chemically precipitated.
High temperature solidification can result from cement contamination in combination with high solids. Since tests at ambient temperature will not reflect this problem, tests which simulate downhole conditions should be run. The Fann Consistometer and Fann Model 50 VG Meter simulate downhole temperature, shear, and pressure conditions and give an indication of solidification tendencies. To determine remedial treatments, it is recommended that pilot testing procedures include hot rolling.
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